Cytonuclear incompatibility contributes to the early stages of speciation

Genetic incompatibility is a hallmark of speciation. Cytonuclear incompatibilities are proposed to be among the first genetic barriers to arise during speciation. Accordingly, reproductive isolation (RI) within species should be heavily influenced by interactions between the organelle and nuclear genomes. However, there are few clear examples of cytonuclear incompatibility within a species. Here, we show substantial postzygotic RI in first‐generation hybrids between differentiated populations of an herbaceous plant (up to 92% reduction in fitness). RI was primarily due to germination and survival, with moderate RI for pollen viability. RI for survival was asymmetric and caused by cytonuclear incompatibility, with the strength of incompatibility linearly related to chloroplast genetic distance. This cytonuclear incompatibility may be the result of a rapidly evolving plastid genome. Substantial asymmetric RI was also found for germination, but was not associated with cytonuclear incompatibility, indicating endosperm or maternal‐zygote incompatibilities. These results demonstrate that cytonuclear incompatibility contributes to RI within species, suggesting that initial rates of speciation could be influenced by rates of organelle evolution. However, other genetic incompatibilities are equally important, indicating that even at early stages, speciation can be a complex process involving multiple genes and incompatibilities.     

Floral trait evolution associated with shifts between insect and wind pollination in the dioecious genus Leucadendron (Proteaceae)

Transitions between animal and wind pollination have occurred in many lineages and have been linked to various floral modifications, but these have seldom been assessed in a phylogenetic framework. In the dioecious genus Leucadendron (Proteaceae), transitions from insect to wind pollination have occurred at least four times. Using analyses that controlled for relatedness among Leucadendron species, we investigated how these transitions shaped the evolution of floral structural and signaling traits, including the degree of sexual dimorphism in these traits. Pollen grains of wind‐pollinated species were found to be smaller, more numerous, and dispersed more efficiently in wind than were those of insect‐pollinated species. Wind‐pollinated species also exhibited a reduction in spectral contrast between showy subtending leaves and background foliage, reduced volatile emissions, and a greater degree of sexual dimorphism in color and scent. Uniovulate flowers and inflorescence condensation are conserved ancestral features in Leucadendron and likely served as exaptations in shifts to wind pollination. These results offer insights into the key modifications of male and female floral traits involved in transitions between insect and wind pollination.     

Complex migration and breeding strategies in an elusive bird species illuminated by genetic and isotopic markers

Unlike the annual bi‐directional movements of over 200 bird species within the Palaearctic–Afrotropical region, irregular movements such as irruptive migration with a low degree of philopatry are reported for a variety of species depending on highly seasonal and unpredictable resources. These flexible movements allow for itinerant breeding – consecutive breeding attempts in two or more geographically different regions during the same annual reproductive cycle. In order to illuminate migratory and breeding strategies of the erratic wetland species Baillon's crake Zapornia pusilla across the W‐Palaearctic–Afrotropical region, we used a set of six DNA microsatellites as well as δ²H_f values of individuals sampled at one African and four European breeding sites. We investigated the degree of genetic population structure within and among different sites and assigned individuals’ feathers of unknown origin to their probable moulting (hence breeding) site using a likelihood approach. We found three genetic clusters, differentiating into one ‘European’ and two ‘African’ populations. Connectivity between the sampling sites was probable as genetic ‘African’ individuals were found in breeding conditions in Europe and vice versa. Likewise, assigned moulting locations based on δ²H isoscapes suggested trans‐continental movements as well as moulting and possibly breeding by the same individual both in African and European breeding grounds. Both isotopic and genetic data reveal the Baillon's crake pursue a complex migration and breeding strategy, allowing as well for irruptive movements and itinerant breeding across the W‐Palaearctic–Afrotropical region. However, a better knowledge about the species’ distribution as well as a more comprehensive data set, including samples from the southern and eastern boundaries of the distribution area would be necessary to improve the spatial resolution to the precision required to unambiguously infer migration directions and extent of exchange between African and European breeding grounds.     

Helicobacter pylori strains vary cell shape and flagellum number to maintain robust motility in viscous environments

The helical shape of the human stomach pathogen Helicobacter pylori has been suggested to provide mechanical advantage for penetrating the viscous stomach mucus layer. Using single‐cell tracking and quantitative morphology analysis, we document marked variation in cell body helical parameters and flagellum number among H. pylori strains leading to distinct and broad speed distributions in broth and viscous gastric mucin media. These distributions reflect both temporal variation in swimming speed and morphologic variation within the population. Isogenic mutants with straight‐rod morphology showed 7–21% reduction in speed and a lower fraction of motile bacteria. Mutational perturbation of flagellum number revealed a 19% increase in speed with 4 versus 3 median flagellum number. Resistive force theory modeling incorporating variation of both cell shape and flagellum number predicts qualitative speed differences of 10–30% among strains. However, quantitative comparisons suggest resistive force theory underestimates the influence of cell body shape on speed for helical shaped bacteria.     

Taxonomic identity determines N2 fixation by canopy trees across lowland tropical forests

Legumes capable of fixing atmospheric N₂ are abundant and diverse in many tropical forests, but the factors determining ecological patterns in fixation are unresolved. A long‐standing idea is that fixation depends on soil nutrients (N, P or Mo), but recent evidence shows that fixation may also differ among N₂‐fixing species. We sampled canopy‐height trees across five species and one species group of N₂‐fixers along a landscape P gradient, and manipulated P and Mo to seedlings in a shadehouse. Our results identify taxonomy as the major determinant of fixation, with P (and possibly Mo) only influencing fixation following tree‐fall disturbances. While 44% of trees did not fix N₂, other trees fixed at high rates, with two species functioning as superfixers across the landscape. Our results raise the possibility that fixation is determined by biodiversity, evolutionary history and species–specific traits (tree growth rate, canopy stature and response to disturbance) in the tropical biome.     

Integrated multi‐omics analysis supports role of lysophosphatidylcholine and related glycerophospholipids in the Lotus japonicus–Glomus intraradices mycorrhizal symbiosis

Interaction of plant roots with arbuscular mycorrhizal fungi (AMF) is a complex trait resulting in cooperative interactions among the two symbionts including bidirectional exchange of resources. To study arbuscular mycorrhizal symbiosis (AMS) trait variation in the model plant Lotus japonicus, we performed an integrated multi‐omics analysis with a focus on plant and fungal phospholipid (PL) metabolism and biological significance of lysophosphatidylcholine (LPC). Our results support the role of LPC as a bioactive compound eliciting cellular and molecular response mechanisms in Lotus. Evidence is provided for large interspecific chemical diversity of LPC species among mycorrhizae with related AMF species. Lipid, gene expression and elemental profiling emphasize the Lotus–Glomus intraradices interaction as distinct from other arbuscular mycorrhizal (AM) interactions. In G. intraradices, genes involved in fatty acid (FA) elongation and biosynthesis of unsaturated FAs were enhanced, while in Lotus, FA synthesis genes were up‐regulated during AMS. Furthermore, FAS protein localization to mitochondria suggests FA biosynthesis and elongation may also occur in AMF. Our results suggest the existence of interspecific partitioning of PL resources for generation of LPC and novel candidate bioactive PLs in the Lotus–G. intraradices symbiosis. Moreover, the data advocate research with phylogenetically diverse Glomeromycota species for a broader understanding of the molecular underpinnings of AMS.